A mold type semiconductor laser is disclosed by, for example, Japanese Patent No. 2951077 and constituted as shown in FIG. 5. In FIG. 5, a sub-mount 7 on which a laser (or LD) chip 8 is bonded is mounted on the die pad portion 2a on the tip end of a common lead 2 among three leads 2, 3 and 4 formed integrally as a lead frame 1. The LD chip 8 and a light receiving element 5 for monitoring are wire-bonded to the other leads 3 and 4 by wires which are not shown. Then, as shown in FIG. 5, a frame is molded by the synthetic resin, e.g., transfer-molded and formed on the periphery of the sub-mount 7 except for a beam emission side, whereby the leads 2, 3 and 4 are integrated with one another and fixed even if the leads 2 to 4 are separated from the lead frame 1.
The semiconductor laser of this type is adjusted together with, for example, the optical axis of a diffraction grating, a collimator lens, a finite objective lens or the like and thereby used as the light source of a pickup device or the like. In this case, as shown in FIG. 5, the sub-mount 7 is composed of a silicon substrate or the like and the light receiving element 5 is incorporated on the surface of the sub-mount 7 on a rear side thereof opposite to the emission surface of the LD chip 8. The output of the LD chip 8 is automatically controlled (APC) while monitoring the output, thereby detecting the pits of the compact disk or the like with constantly fixed output.
As stated above, if APC is conducted while monitoring the output of the laser chip using the light receiving element and the atmosphere temperature varies with temperature rise due to laser operation, then the output of light received by the light receiving element varies. It is detected that, for example, if the atmosphere temperature is changed while making laser output fixed, detection output is periodically changed. Due to this, it was discovered that even if the output of the laser chip actually has no change, the output detected by the light receiving element varies with temperature and that if the laser output is controlled based on the varying output, the laser output cannot be made fixed.